Wavelength division multiplexing (WDM) systems typically comprise multiple separately modulated laser diodes at the transmitter. These laser diodes are tuned to operate at different wavelengths. When combined in an optical fiber, the WDM optical signal comprises a corresponding number of spectrally separated channels. Along the transmission link, the channels are typically collectively amplified in gain fiber, such as erbium-doped fiber and/or regular fiber, in a Raman pumping scheme. At the receiving end, the channels are usually separated from each other using thin film filter systems, to thereby enable detection by separate photodiodes.
The advantage of WDM systems is that the transmission capacity of a single fiber can be increased. Historically, only a single channel was transmitted in each optical fiber. In contrast, modern WDM systems contemplate hundreds or thousands of spectrally separated channels per fiber. This yields concomitant increases in the data rate capabilities of each fiber. Moreover, the cost per bit of data for WDM systems is typically less than comparable non-multiplexed systems. This is because any amplification system required along the link can essentially be shared by all of the separate channels transmitted in a single fiber link. With non-multiplexed systems, each channel/fiber would require its own amplification system.
Nonetheless, there are challenges associated with implementing WDM systems. First, the transmitters and receivers are substantially more complex since, in addition to the laser diodes and receivers, additional optical components are required to combine the channels into, and separate out the channels from, the WDM optical signal. Moreover, there is the danger of channel drift where the channels loose their spectral separation and overlap each other. This interferes with channel separation and demodulation at the receiving end.
In order to ensure that proper guard bands are maintained between adjacent channels and to also ensure that the carrier frequencies or wavelengths of the channels are correct both relative to other channels and relative to their wavelength assignments, optical monitoring systems are required in most WDM transmission systems. They are also useful in WDM channel routing systems, such as add/drop multiplexers and switches to ensure that the specific optical channels are being property controlled. Further, information concerning the relative and absolute powers in the optical channels is important as feedback to variable attenuators, for example.
Historically, however, optical monitoring systems have been relatively large, complex systems. Their size and complexity, and resulting maintenance requirements, prevented them from being integrated into systems offering high levels of functionality such as cross-connect switches, amplifier systems, and integrated receivers, monitoring systems and transmitters, for example.
The present invention concerns an optical monitoring system that is capable of being integrated into a small package to be used as a subsystem, or possibly even as a stand-alone system, in a WDM system, or other application requiring optical spectral monitoring.
In general, according to one aspect, the invention features an integrated optical monitoring system. It comprises a hermetic package and an optical bench sealed within the package. An optical fiber pigtail enters the package via a feed-through to connect to and terminate above the bench. A tunable filter, connected to the top of the bench, filters an optical signal transmitted by the fiber pigtail. A detector, also connected to the bench, detects the filtered signal from the tunable filter. Thus, the entire system is integrated together, on a single bench within a preferably small package. This configuration makes the system useful as a subsystem, for example, in a larger system offering higher levels of functionality and optical signal processing capability.
In the preferred embodiment, an isolator is also integrated onto the bench to prevent back reflections into the fiber pigtail.
The preferred embodiment uses a reference signal source, also preferably integrated on the optical bench that generates a reference signal, which is filtered by the tunable filter. Such a reference signal enables absolute measurements of optical signal wavelength to ensure that each optical signal is broadcasted at the proper wavelength and to detect such problems as wavelength drift across all of these signals. As a result, the system is capable of detecting absolute frequency, in addition to ensuring that guard-bands are maintained between adjacent channels, for example.
In the current embodiment, the reference signal source comprises a broadband source and an etalon. The etalon converts the broadband signal from a super luminescent LED (SLED), for example, into a signal with stable spectral characteristics.
In other embodiments, two physically discrete tunable filter cavities are utilized. Typically, the cavity tuning is synchronized to obtain net signal transmission through both cavities.
In general, according to another aspect, the invention is also characterized as a method for constructing an integrated optical monitoring system. This method comprises installing an optical bench in a hermetic package. A fiber pigtail is inserted through a fiber feed-through, into the package, and terminated on the optical bench. A tunable fiber is also installed on a top of the bench to filter an optical signal from the fiber pigtail. Finally, a detector is installed on the bench to detect the filtered optical signal from the tunable filter.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.